Throughout this disclosure, including in the claims, the expressions "processor" and "means for processing" are employed to denote devices within the broad class of data recording apparatus, data processing apparatus, and apparatus for providing control signals to a peripheral device (such as a sensor apparatus). Examples of "processors" of this type include desktop computers, networks of computers, and devices such as printers, magnetic disk storage devices, and digital electronic memory banks. The invention provides a wireless interface between a sensor apparatus (such as a multimeter for performing electrical measurements) and one or more "processors."
Throughout this disclosure, including in the claims, the expression "meter" is employed to denote any device which directly measures or monitors electrical parameters such as voltage or current, or any measuring or monitoring device which includes one or more transducers (or sensors) for converting physical quantities into electrical parameters (for subsequent processing).
Also throughout this disclosure, including in the claims, the phrases "transmit radiation" and "transmission of radiation" are used in a broad sense, to denote original transmission of radiation from an emitter, and also modulation of previously emitted radiation. For example, "transmission of radiation" is used to denote modulation of a radiation beam (to generate a modulated beam which transmits encoded data). Also, the words "signal" and "signals" are used throughout this disclosure (including in the claims) to denote not only data and representations of magnitudes, but also representations of control parameters such as range and function settings and instructions.
Throughout science and industry there is a perpetual need to sample, record, and process data on the magnitudes of various physical quantities, such as voltages, temperatures, pressures, currents, magnetic fields, light intensities, acidities, and so on. The quantities of interest may be interdependent, they may vary with time, or they may differ instead from one unit under test to another. If the quantity to be metered is not a voltage, then a sensor or transducer (such as a thermocouple) converts the magnitude into an electronic signal (typically an analog voltage) which represents the quantity. It is these electronic signals (analog voltages) which are acquired as data, through the use of instruments such as voltage meters, multimeters, instrumentation amplifiers, analog-to-digital acquisition devices, and the like.
As their name implies, multimeters are capable of measuring several electronic quantities: voltages, resistances, capacitances, currents, frequencies, etc. They are usually hand-holdable, battery-powered devices and are often used with test leads (wires that terminate in probes which are touched to or temporarily clipped to the source which is under test).
Multimeters are often used by technicians who are diagnosing problems in pieces of equipment which are defective. Under these circumstances the multimeter's probes may be accidentally touched to very high voltages--either because the equipment is defective in such a way as to cause junctions which are normally at low voltages to be instead at very high voltages, or, because the technician simply makes an error in analyzing a rather unfamiliar circuit.
Typically, conventional meters (including multimeters) have displays which exhibit the current value of the reading; some can even temporarily store the measured values. Conventional multimeters are "handy" (in the sense that they fit in the hand), are powered by batteries, and their various functions and ranges are selected by one or more dials or switches on a front panel.
The displays are usually liquid crystal displays (LCD's) which work by reflection of ambient light. Some LCD's are backlit, but these consume power and so are not often used on small battery-powered devices, although they have been so used. It is often necessary, in order to read an unlit display, to deliberately orient the meter in such a way as to catch the light. If the meter is being used at a station which has no available surface or mounting for the meter which would allow it to be oriented in the preferred direction, then the operator may need to temporarily hand-hold the meter in order to angle the display toward the light. If it is dark then a flashlight or other external source of illumination may be necessary. In some working environments where it is necessary to avoid such general illumination of the work area, it is difficult to use meters which lack lighted displays.
Most conventional meter displays are digital, and some digital meters or instrumentation amplifiers are equipped with a communications port which allows them to be connected to a computer through a cable. For example, a product available from Exteck Instruments Corporation (of Waltham, Mass.) consists of a digital multimeter which has a built-in RS232 port. When the meter is connected to a computer by a cable to its RS232 port, sampled values of voltage, current, capacitance, resistance, or the like, may be acquired by the computer for storage and processing.
A computer-connected multimeter can be a cost-effective means of gathering statistical process control data on electronic components (i.e., for quality assurance applications). A computer program could be written which would lead an unskilled operator through a series of readings as a circuit board is probed. The program could determine if and where the board is defective and could store the readings for subsequent statistical analysis of failure modes. Complicated and expensive automatic testing equipment is already available for these tasks, but the multimeter-plus-desktop-computer combination offers advantages: because probe contacts are manually made and because the computer is programmable there is flexibility; the cost is low; technical service personnel often have prior familiarity with both multimeters and with the operating systems of desktop computers.
For many years there have been "output" connectors or jacks on many types of meters and 5 instrumentation amplifiers to enable conductive cables or wires to transport the output to recording devices. Recently, output connectors have been provided for supplying digitized data directly to digital computers. In all such digital products which have output connectors for wires or cables, the connector must be supported by internal circuitry which meets some digital communications port specification, such as the "RS232" or "IEEE488" specification, for driving the digital signal down the wire (or cable).
Of course a user of a meter (having an output connector for a wire or cable) may just want to take note of the readings from the display and may never need to connect the output connector to a computer, in which case the connector itself and the associated additional circuitry add cost and bulk to the meter without giving any advantage to the user. A meter manufacturer could offer two models of a meter, one with and one without a communications port, but that course has obvious drawbacks: production is thereby complicated, vendors may not wish to carry a higher-cost model if it is not a big seller, and a user may buy the less expensive meter without the port only to later determine that the port would have been useful.
There are other obvious drawbacks to connecting a multimeter (or other meter) to a processor via a cable. Some of the "handiness" is immediately lost, since the meter is virtually tied down by the cable. Of course the cable can be disconnected and reconnected, but the required connectors for digital communications are typically multi-pin connectors and so care is needed to properly align the connectors when they are reconnected. When repetitive tasks are to be performed, even this minor additional labor is likely to prove to be a significant hinderance.
Moreover, there is the matter of care of the cable when it has been disconnected from the meter but is still connected to the computer. The above-mentioned Exteck Instruments Corporation meter makes use of the very same kind of jack which is found in so-called "modular" telephones. The four pins of the male jack are pressed against the corresponding contacts in the female connector within the meter by a plastic tang which is part of the male jack. The tang is fully exposed and is easily broken off, as when the dangling cable is allowed to rest on the floor and someone steps on it. Of course a more robust type of connector could be used, but there are associated problems of bulk and cost. Stronger connectors which have pins recessed and shielded by a metal band or cylinder are more costly since the construction involves an additional metal part which must be separately fabricated and then joined to the plastic body (whereas modular telephone jacks are of a simple all-plastic, one-piece construction). Furthermore, any connector has some bulk to it, which is undesirable since compactness is among the most desired qualities of hand-holdable meters.
Some types of electronic components (e.g., power-efficient "CMOS" logic devices) are very easily harmed by commonplace levels of static electricity, such as might be transmitted from the user's finger to one of the pins of the jack on the cable as it is connected or disconnected. Thus, some precautions must be taken in order to guard against damage to the active electronic hardware which is used to send and receive the digitized signals to and from the communications port. The available precautions consist of either the addition of protective components or the use of components which are inherently static-resistant but which would presumably otherwise be considered to be second-best (e.g., TTL logic devices are more static-resistant than CMOS but they are also more power-hungry).
It is also possible that the dangling cable could be caught up in moving machinery.
The Fluke Manufacturing Corporation has offered products known as the models 93, 95 and 97 "scopemeters," which are similar, and of which the model 97 has a digital communications port. These products are portable digital storage oscilloscopes and multimeters with liquid crystal displays. The model 97 has an optical port. To use the port, it is necessary to attach a cable which contains an optical-to-electrical converter to the scopemeter.
Optical isolation (e.g., optical isolation of output terminals from the rest of the internal circuitry as in the Fluke Corporation model 97 scopemeter) would be a highly desirable feature for a multimeter which could be connected to a computer, especially if it could be provided for little cost. Consider, for example, the case that the basic requirement is to measure a small difference between two rather large voltages (where both voltages are well above "ground"). If in this case the voltage drop to be measured has a "dc" component or is of low frequency, then capacitive coupling of the inputs would not suffice. Suppose that the input probes of the multimeter are affixed to two junctions on some source under test which are both at elevated voltages (i.e., they are elevated above "ground") and the "low" voltage pin on the cable from the computer is, by design or necessity, at ground. Of course the desired reading is the voltage across the input terminals, the difference between the two elevated voltages. However, if one of the output terminals is grounded by the connection to the computer, and is not electrically isolated from the internal circuitry which measures the voltage difference across the input terminals, then the meter reading may be incorrect.
The characteristic of a meter which has internal circuitry to overcome this problem is usually called "common-mode rejection", the minimization of the effect of the component of the voltages on the input terminals which is common to both terminals upon the reading. If a "direct-coupled" meter lacking optical coupling has internal circuitry which provides for a substantial measure of common-mode rejection, the rejection capabilities will be limited. For example, if one of its output terminals is grounded then the meter reading will be erroneous if the component of the voltage which is common to both terminals is higher than the voltage of the meter's batteries (in effect, the common-mode rejection feature of the circuitry uses the meter's batteries to "buck-out" the common-mode voltage). Optical coupling would break the circuit between the input terminals and the output terminals, and so would enable almost any common-mode voltage to be rejected because there would be no grounding, so the common-mode voltage would not be not applied to the circuitry.
However, at very high common-mode voltages even monitoring instruments with internal optical coupling can fail, as when the common-mode voltage is so high as to cause a spark to jump across the internal optical coupling device. Thus, a scopemeter with internal optical coupling is limited by the breakdown voltage of its internal optical coupling device to an operating range which does not extend to very high source voltages. A greater degree of electrical isolation than is provided by an internally optically-coupled meter would therefore be desirable, particularly for a multimeter, because connecting a multimeter (or other meter) to a computer through an optical-coupling link which could break down would create the risk that a computer operator (who of course does not expect to encounter the risk of high voltages at the computer) could be shocked. Voltages insufficient to injure the computer operator could nonetheless cause extensive damage to computer circuitry, or loss of data, or both. An important advantage of the present invention is to avoid the exposure to such risks, thereby entirely escaping the attendant product liability, risk control, and regulatory burdens.
Optical coupling also eliminates ground loops. When a voltage source under test is connected to a meter (which measures and/or records data) through wires or cables, and the meter circuitry is not isolated from ground (as by an optically-coupled link), then there are two kinds of paths between the source under test and the meter: the intended paths through the wires or cables; and "ground" paths (through desktops, floors, the earth, or plumbing in the walls, to name some examples). The ground paths and the wires or cables form closed "ground loops".
According to Faraday's law of electromagnetism, a time-varying magnetic field through a ground loop (such time-varying magnetic fields are generated by radio signals and by alternating current in power mains) generates a voltage drop around the loop. Some fraction of the voltage drop appears along the lengths of the wires or cables, because they have non-zero resistances. This voltage drop is sensed by the measuring or recording device as a spurious signal superimposed upon the "true" voltage difference to be measured.
An optical link would break such ground loops. When a ground loop is broken, no current is induced therein, and so no spurious voltage drop appears along the wire or cable (thus preventing transient currents on power lines and radio waves from appearing as spurious signals). There are other ways of reducing the effects of ground loops, such as to enclose the leads in a conductive sheath or to twist the leads together so as to cause the spurious ground loop voltage to be equally present on both leads and hence not evident as a voltage difference between the leads. These are effective measures, but not as effective as use of optical links (which are nearly 100% effective).
There is another way that optical links are used for communications with computers. For several years the Hewlett-Packard Corporation has manufactured and sold "hand-held computers" and calculators which broadcast an infrared beam and which can thereby communicate digitally (via an optical link) with certain peripheral devices. To use the optical link it is necessary deliberately to aim the port of the hand-holdable computer or calculator at the corresponding port of the peripheral device. Most commonly the peripheral device is a printer, but a battery-powered, multi-channel, analog-to-digital converter and input-output control device (model ADCM-48) available from EduCALC Corporation of Laguna Niguel, Calif., has an infrared port for communication with a Hewlett-Packard Corporation hand-held computer. The model ADCM-48 is not a portable "meter" with a display and controls for selecting voltage ranges; it is a box with terminals for analog voltage inputs and digital control inputs and outputs.
With all of these Hewlett-Packard (or Hewlett-Packard-compatible) peripheral devices, in order to use the infrared link one must position the computer or calculator within a foot or two of the peripheral device and aim it in the general direction of the peripheral device. Usually this requirement is met by placing both devices on the same level surface, such as a benchtop, desktop, or floor. If the inter-device distance is too great, then the communication will be garbled or will not occur at all. Of course, the line of sight between the devices cannot be intercepted by any obstacle, as that would block communications.
Such optical links as are found on the Hewlett-Packard or Hewlett-Packard-compatible devices offer the general advantages of optical links which are described above, but the positional use limitations which encumber such links would be severe if they were to be imposed upon many types of meters (such as multimeters). Even if a computer for receiving signals broadcast over such an optical link is hand-holdable, it is undesirable to require always a clear spot on a level surface for the computer just to effect the optical linkage. Furthermore, because this kind of optical link requires aiming of the single port on the computer, it is obviously not adaptable to simultaneous communication with several peripheral devices. Furthermore, with this kind of optical link it is only feasible to transmit data and control signals. It would be advantageous to be able also to transport light over an optical link for the illumination of an LCD display of a peripheral device.
Other conventional "wireless" computer networks work by broadcasting infrared signals between multiple desktop computers and peripheral devices (for example, in open-plan offices). In order to work, they require a continuously open path for the infrared radiation. Furthermore, these networks can transmit only data and control signals, not light for illumination.
Another other class of conventional products uses infrared beams to transmit data and control parameters: remote controls for television sets, videocassette recorders, and audio equipment. Some such systems have interesting and relevant advanced capabilities. For example, at least one such remote controller is "intelligent" in the sense that it can read and "learn" the infrared transmissions of controllers produced by other manufacturers. In this way it can be programmed to be a universal controller for many remote entertainment appliances of different manufacturers (to each it sends appropriately-coded control parameters). This is a capability which is made practical by the very nature of data transmission on a beam of light rather than on a cable: there are no hookup-related hardware incompatibilities (connectors, pin diagrams, voltage levels, etc.) to deal with.
Because the Hewlett-Packard and Hewlett-Packard-compatible devices, and conventional television remote control devices and the like, are portable and communicate with infrared radiation over distances of a foot or more, they use beams which are fanned-out in order to avoid having to point their ports precisely at each other to effect the linkage. The receiving ports are but a fraction of an inch across, and so only a small fraction of the broadcast radiation actually enters the receiving ports and is used to effect the linkage. Thus, because they lack a means for ensuring that most of the radiation from one port reaches another, such devices necessarily waste much of the radiated infrared power. This wastage is tolerable for television channel changes (which are very brief and isolated events) or occasional sessions in which a computer or calculator is linked to a printer, but constant data transmission over such a link would tax the batteries of the portable device.
Until the present invention, it was not known how to eliminate the described disadvantages and limitations of conventional apparatus for coupling a meter with a computer by means of a wireless link.
The present invention provides numerous advantages over the prior art. For example, the invention:
allows several meters (rather than just one meter) to exchange data and control parameters with a processor (such a computer, computer network, or data recording device); PA1 provides an extreme degree of electrical isolation between a processor and one or more meters for the complete safety of personnel, data, and equipment, and to reject common-mode voltages and ground-loop noise (this enables the invention to be employed to take measurements in a variety of electrically hostile environments); PA1 incorporates a wireless (connector-free) communications link within a receptacle or mounting means (station) for a meter, which link is easily and safely broken when the meter is temporarily freed from its station for other use, thus avoiding the mechanical hazards of dangling cables, the need to implement the linkage with static-resistant parts or circuits, and the labor of making and breaking the contacts of a multi-pin connector every time the meter is removed from its station; PA1 provides light for illumination of a meter's display, without drawing on batteries within the meter or using any connection to the meter, thus making the display easy to read in any light and avoiding the need to tip the display toward the available ambient light every time it is read; PA1 causes wireless communication to occur a "point-blank" range, thus making minimal use of power and prolonging the battery life of the meter; and PA1 optionally allows wireless communication to occur without the meter being required to generate signal-transporting radiation, thus making minimal use of power and prolonging battery life.
Further advantages of the invention follow from the linkage with computers and other data processing or recording equipment which the invention naturally accommodates. With the invention, meters can be daisy-chained together, thereby enabling nearly simultaneous serial communications between several such meters and a processor (such as a computer, printer, recording apparatus, or computer network). Consequently, the invention allows an otherwise-ordinary multimeter and a desktop or portable computer (having an internal clock) to replace a strip chart recorder, and two multimeters and a computer to replace an "x-y" recorder and display the relationship between two interdependent magnitudes.
This distributed multi-channel capability is different from that offered by analog-to-digital converters which have the multi-channel capability within a single instrument. Signal-level voltages cannot be sent over very long wires or cables unless first amplified in order to overcome electrical noise and the resistances of the wires or cables. Hence, for successful operation, each source of voltages to be measured must be rather close to the inputs of the measuring device. Thus, the multi-channel feature of analog-to-digital converters (such as the above-described model ADCM-48) will not be usable with widely-separated sources of signal-level voltages. In contrast, the multi-channel capability of the invention, allows each linked ("on-line") meter to operate as a channel and to be placed as close to a voltage source (or sources) as necessary. Hence, each voltage source may be at any distance from each other voltage source and from the processor (i.e., at any distance feasible for the operation of the digital network). This is not possible with the ADCM-48 scheme because the infrared beam from the computer must be pointed directly at the ADCM-48 from a nearby location, and the beam can only be pointed in one direction at a time.
The linkage provided by the invention is also immune to spurious interference from other digitally-communicating devices which may afflict other types of data links. For example, conflicts may arise between systems which broadcast infrared radiation around a room if two different systems by two different manufacturers are used at the same time. Some embodiments of the invention have this immunity even if beams of infrared or visible radiation are the means of transmission.
Among the benefits of the invention to a meter manufacturer are many leveraging opportunities. To elaborate on comments made above, the invention requires that only a slight modification or addition be made to a meter in order to make the meter digitally linkable ("network ready"). The cost and effort of incorporating the modification (or addition) would be so small that the meter manufacturer could sell the enhanced product at almost no additional cost to customers who want the linkage capability and to customers who do not. The controller/receiver ("cradle") of the invention, which is the interface between the meter and a processor or which incorporates a processor, could be produced by manufacturers other than the meter manufacturer. Other meter manufacturers could produce meters which use the data and control parameter encoding protocol implemented by the cradle, so that such meters would be compatible with the previously-produced cradle.
There is pre-existing software for data acquisition which would also provide leverage. For example, LabView software, available from National Instruments is designed to work with a variety of data acquisition equipment. It and other software packages already on the market could be modified to accommodate users of the invention, so that meter manufacturers need not develop their own software.
Similarly, the cabling and other hardware which would connect the inventive cradles to processors could be "off the shelf" items which are already available. The invention could be implemented on any of several conventional computer networks or busses. No special transducers or other electronic components are needed to make meters and cradles which implement the invention.
The invention allows meters such as multimeters to be used for new applications. With the inventive cradle and a desktop computer, they can replace strip-chart recorders and x-y plotters. Many older but still-functioning pieces of electronic test equipment have analog outputs (not digital outputs). There are conventional amplifiers of various types with sophisticated inputs for very small signals, and preamplifiers, all with analog outputs. With the invention, voltage meters or multimeters can be attached to such analog outputs, thus converting them to digital outputs, and allowing the output data to be stored, displayed, and analyzed on a computer.
All these possibilities for expanding sales of meters could be captured by a meter manufacturer for just the small expenditure needed to effect the slight modification or addition which makes a meter optically linkable with the inventive cradle.
Throughout the remaining portion of this disclosure, including in the claims, the term "radiation" is employed in a broad sense to denote any type of radiation or field that is capable of being modulated so as to transmit data or other signals across a wireless communication link. Examples of such "radiation" include electromagnetic radiation (such as, but not limited to, visible, infrared, or ultraviolet radiation), acoustic radiation, the field surrounding a capacitive or inductive coupling device, and the near field of an acoustic source.